The California Air Resources Board (CARB) mandates that vehicles powered by an internal combustion engine must include onboard Diagnostic systems to monitor the operation of the internal combustion engine and other components to ensure ongoing vehicle compliance with air quality standards.
The onboard diagnostic systems begin operation each time the vehicle is started. In order to track the performance of the onboard diagnostic systems and verify that the various vehicle systems and/or components are being tested, and that the onboard diagnostic systems are completing their tests, each vehicle includes a control algorithm that tracks a ratio of the number of times the onboard diagnostic system tests are successfully completed to the number of times minimum criteria are met, sometimes referred to as “Standard Conditions Met” (SCM) criteria. This may be referred to as the “N/D ratio”. Every time the SCM criteria, described in greater detail below, are satisfied, the denominator “D” is incremented. Every time the onboard diagnostic systems successfully complete their diagnostic checks or time sufficient to identify a failing diagnostic check has passed, the numerator “N” is incremented. Accordingly, if the onboard diagnostic systems complete their diagnostic checks every time the SCM criteria are satisfied, then the N/D ratio would equal one (1). For example, if the SCM criteria are satisfied one hundred times (D=100), and the onboard diagnostic systems complete the diagnostic checks each of those occurrences (N=100), then the N/D ratio is equal to 100/100, which is equal to 1. However, if the onboard diagnostic systems complete their diagnostic checks only half the time the SCM criteria are satisfied, then the N/D ratio would equal 0.5. For example, if the SCM criteria are satisfied one hundred times (D=100), and the onboard diagnostic systems complete the diagnostic checks only half of those occurrences (N=50), then the N/D ratio is equal to 50/100, which is equal to 0.5. If, upon the next drive cycle wherein the SCM criteria are satisfied and the diagnostic check does not successfully complete, the numerator N does not increment (N remains equal to 50), but the denominator D does increment (D=101 and the N/D ratio is equal to 50/101. The N/D ratio must remain over a pre-defined level to ensure proper functioning of the onboard diagnostic systems and satisfy the CARB requirements. For example, the N/D ratio for each onboard diagnostic system typically must remain over 0.333 to satisfy the CARB requirements.
As noted above, the vehicle must satisfy certain minimum criteria before the “D” of the N/D ratio may be incremented, i.e., the SCM criteria. However, even if the vehicle satisfies the three minimum criteria to increment the D of the N/D ratio, the onboard diagnostic systems may not have enough time to complete their diagnostic checks of the various systems and/or components, which would prevent the numerator N from incrementing. These minimum criteria include a total time criteria, a vehicle speed criteria and an idle criteria. Accordingly, once the vehicle is started and operation of the onboard diagnostic systems has begun, the vehicle must remain on for a total time of six hundred seconds (600 sec) to satisfy the total time criteria, the speed of the vehicle must remain over twenty five miles per hour (25 mph) for at least three hundred seconds (300 sec) to satisfy the vehicle speed criteria, and the vehicle must remain at idle, i.e., a vehicle speed equal to zero miles per hour (0 mph), for at least thirty seconds (30) to satisfy the idle criteria. If the vehicle is turned off prior to all three of these criteria being satisfied, then the D of the N/D ratio does not increment.
Satisfying the N/D ratio requirements under the CARB mandates is usually not a problem for traditional vehicles that are powered only by internal combustion engines, i.e., the internal combustion engine is always operating when the vehicle is turned on, providing sufficient time for the vehicle to satisfy the three minimum criteria to increment the D of the N/D ratio and allow the onboard diagnostic systems to fully complete their diagnostic checks. However, for plug-in electric hybrid vehicles, meeting the N/D ratio requirements becomes more difficult. This is because the vehicle may be turned on and moving under electric power, without ever starting the internal combustion engine. If the vehicle is operating under electric power and the internal combustion engine does not start until a period of time into the trip, the three minimum criteria may be satisfied and the D of the N/D ratio is incremented, without the onboard diagnostic systems having sufficient time to complete their diagnostic checks, which leads to the numerator N of the N/D ratio not incrementing. Accordingly, the denominator D of the N/D ratio increments, whether the vehicle is operating under electric power or powered by the internal combustion engine, but the numerator N of the N/D ratio does not increment because of insufficient time remaining in the drive cycle to complete the diagnostic check. This leads to a low N/D ratio, which may bring the plug-in electric hybrid vehicle out of compliance with the N/D ratio requirements under the CARB mandates.